Apical Na+/H+ exchange is a key regulatory step in controlling the conservation of salt and water in the large intestine, and when its function is perturbed, diarrheal disease can result. Our long-term objective is to determine the cellular mechanisms by which apical Na+/H+ exchange is regulated in the colonic epithelium. Recent studies suggest that both NHE2 and NHE3 isoform Na+/H+ exchangers have a role in colonic Na+ absorption. While abundant evidence supports a role for NHE3 in intestinal and renal sodium absorption, less is known about NHE2 function and regulation. Preliminary data from both mouse crypt colonocytes and HT29-C1 tissue culture cells suggest that apical NHE2 function is activated by short-chain fatty acids (SCFAs), endogenous compounds in the colonic lumen that are major non-endocrine regulators of Na+ absorption. The goal of this proposal is to test the concept that both NHE2 and NHE3 are regulated by translocation to/from the plasma membrane in response to SCFAs or regulatory agonists.